The production of printed circuit boards typically employs one or more steps wherein a photoimageable composition is utilized. Primary photoresists are used to form the initial circuitry pattern by processes such as etching or additive plating. Once the circuitry traces are formed, a printed circuit board is typically covered with a solder mask that is formed from a photoimageable composition which is hardenable so as to form a permanent layer covering circuitry on a printed circuit board.
Photoimageable compositions known in the art may be positive-acting or negative-acting. Positive-acting resists comprise a polymeric resin and a sensitizer. Upon exposure to actinic radiation the sensitizer effects degradation of the polymeric resin in the exposed areas. The degraded polymeric resin is then washed away by developer. Negative-acting resists comprise a film-forming binder, a photopolymerizable material (which may or may not include chemical moieties of the binder), and a photoinitiator or photoinitiator chemical system. Upon exposure to actinic radiation, a photoinitiated polymerization of the photopolymerizable material is initiated, resulting in immobilization of the film-forming binder and thereby causing the exposed portions to be insoluble in developer, such as a solvent developer or an aqueous based developer.
Examples of various types of photoimageable compositions are found in U.S. Pat. Nos. 3,953,309, 4,064,287, 2,670,287, 3,376,139, 3,402,044 and 5,229,252, the teachings of each of which are incorporated herein by reference.
For forming films or layers of photoimageable compositions on a substrate, such as a primary resist layer on a circuit board blank, a solder mask-forming layer on a printed board, or as a layer on a support sheet for forming a dry film, it is most common to dissolve the components of the photoimageable composition in a solvent or solvents, apply the photoimageable composition to the substrate as a liquid, and then allow the solvent to evaporate to form the film or layer. A variety of processes of applying the liquid composition are available, such as spraying, dip coating, roller coating, or curtain coating. The choice of solvent or solvents affects not only the drying time of the applied photoimageable composition but the characteristics of the applied film. Rapid drying is generally desirable from an efficiency standpoint; however, if drying is too rapid, solvent bubbles may develop.
Rapid, uniform application of a photoimageable composition to a substrate may be done by curtain coating wherein a substrate, such as a printed circuit board, is conveyed through a falling curtain of photoimageable composition, e.g., a solder mask-forming resist. Such a process is taught in U.S. Pat. No. 4,230,793, the teachings of which are incorporated herein by reference. In a curtain coating operation, drying of the coated boards is effected by conveying the coated boards through a drying oven, e.g., at temperatures of between about 85.degree. and about 240.degree. C. The drying time of the photoimageable composition is particularly critical in such a continuous process.
The present invention is directed to a solvent system for photoimageable compositions which provides a particularly desirable dry time. The solvent system of the invention is suitable in all of the types of photoimageable compositions discussed above and suitable for all methods of application.
The solvent system of the present invention is particularly suitable for negative-acting photoimageable compositions and most particularly suitable for solder mask-forming photoimageable compositions containing an epoxy resin for post exposure/post development hardening. Such photoimageable compositions are found for example in above-referenced U.S. Pat. No. 5,229,252 and U.S. patent application Ser. No. 08/075,356 filed 10 Jun. 1993, now U.S. Pat. No. 5,364,736, the teachings of each of which are incorporated herein by reference.
The solvent system of the present invention is particularly advantageous for photoimageable compositions which are applied by curtain coating. Examples of commercial epoxy resin-containing, solder mask-forming, photoimageable compositions are Epic.RTM. CC-100 and Epic.RTM. CC-200 sold by Morton International, Morton Electronic Materials.